Iron-57 NMR chemical shifts and mössbauer quadrupole splittings in metalloporphyrins, ferrocytochrome c, and myoglobins: A density functional theory investigation

We have evaluated the ⁵⁷Fe nuclear magnetic resonance chemical shielding and Mössbauer electric field gradient tensors and their orientations for a cytochrome c model compound as well as for isopropyl isocyanide and carbon monoxy-myoglobin model systems and two simple metalloporphyrins containing bis(pyridine) and bis(trimethylphosphine) ligands, using Kohn-Sham density functional theory. For cytochrome c we used a model Fe(II) porphyrin structure together with a 1-methylimidazole base (to represent His-18) and a dimethyl sulfide molecule (to represent Met-80 in the structure of horse heart ferrocytochrome c), both located at the X-ray coordinates for cyt c Fe(II). For the Mb calculations, we used the coordinates of two recently characterzed metalloporphyrins: (i-PrNC)(1-methylimidazole)(5,10,15,20-tetraphenylporphinato)Fe(II) and (CO)(1-methylimidazole)(5,10,15,20-tetraphenylporphinato)Fe(II), while literature structures were used for the bis-ligand adducts. We used a "locally dense" basis to evaluate the ⁵⁷Fe shieldings and electric field gradients at iron and compared them with the measured chemical shifts and Mössbauer quadrupole splittings, respectively. There is moderately good agreement between theory and experiment for the cytochrome c and Mb ⁵⁷Fe chemical shifts and shielding tensors, and ten good (0.10 mm s^-1 rmsd) agreement for the ⁵⁷Fe Mössbauer quadrupole splittings, using the following basis sets and functional: a Wachters all electron representation for iron, a 6-311++G(2d) basis for all atoms directly attached to iron. 6-31G* for the second shell and 3-21G* bases for the other more distant atoms, together with a B3LYP hybrid exchange-correlation functional. Extensive tests with other functionals and basis set schemes are also reported. The shift and electric field gradient tensor orientations are generally close to obvious molecular symmetry axes, with the skew of the shielding tensor reversing sign on transition from strong to weak ligand fields. The paramagnetic contribution to shielding overwhelmingly dominates overall shielding and the variations seen between weak ligand field (bis(pyridine), cytochrome c) and strong ligand field (CO, PMe₃, i-PrNC) systems. Poor accord between theory and experiment is obtained for the ⁵⁷Fe chemical shifts when MbCO models having highly distorted X-ray geometries are employed, suggesting that the Fe-C-O is close to the porphyrin normal, both in solution and in the solid state.